The present invention relates to a development unit for a reproduction apparatus for the selective application of marking agents to an image-forming medium comprising: a first reservoir for storing marking agents, a second reservoir for maintaining a working stock of marking agents, transfer means for the selective application of marking agents present in the second reservoir to the image-forming medium, feed means for feeding marking agents from the first reservoir to the second reservoir based on a feed control signal, at least one sensor for the generation of a sensor signal corresponding to the quantity of marking agents present in the second reservoir, and control means for generating the feed control signal on the basis of the sensor signal. The present invention also relates to a reproduction apparatus provided with the development unit.
In a reproduction apparatus, the development unit ensures that marking agents are applied to an image-forming medium. In the case of inkjet, this means the application of ink directly on to the copy material. In the case of electrophotography it involves applying toner to a photoconductor, after which the toner image formed on the photoconductor is transferred to the copy material. If, in the case of electrophotography, the "binary" development system is used, then the marking agents are in the form of a toner powder, with the toner powder being contained, for development purposes, in a development mixture together with carrier particles. During operation, this development mixture is continually maintained in movement, so that the toner particles are charged up tribo-electrically by friction with the carrier particles. A magnetic brush then brings the tribo-electrically charged toner particles into the direct vicinity of the photoconductor, where the toner particles leap over selectively, in accordance with the charge image on the photoconductor, so that a toner image corresponding to the charge image is formed on the photoconductor.
During the production of prints, toner will be consumed from the development mixture so that the quantity of toner in the development mixture and hence the toner concentration of the development mixture decreases. To obtain good prints it is essential that the variation in toner concentration should remain within specific limits. To achieve this, it is known in the prior art to keep the toner concentration at a required value by means of a toner concentration control system which controls the toner supply from a reservoir to the development mixture. However, this objective is only partly achieved. In the case of development units discussed hereinabove, in which marking agents are supplied from a reservoir to a working stock from which the marking agents are withdrawn for development purposes, there is always the risk that the situation may occur where the instantaneous consumption of marking agents exceeds the supply so that a shortage of marking agents occurs in the working stock. In the case of the binary process this means that if more toner is consumed than can be supplied during a specific period of time, the toner concentration falls off. If this fall-off is considerable, the print quality is reduced and when a specific critical bottom limit of toner concentration is reached, soiling and also damage of parts may occur.
To prevent such a critical bottom limit from being reached, it is known in the prior art to go over to a delayed-print mode if the toner concentration reaches a certain threshold value. In this mode the printing operation is temporarily interrupted. All the prints in progress are finished but subsequent prints are no longer accepted. The toner supply continues in the usual way so that the toner concentration can be restored. When the toner concentration has again reached a nominal value, then the flow of prints is put in progress. However, this step does not offer any solution to situations in which the instantaneous toner consumption is so high that the critical bottom limit at which soiling and damage of parts occurs is reached during the finishing of the current print. In order to minimise the negative consequences of such situations, it is known in the prior art to stop printing directly if the critical bottom limit is exceeded, for example by switching of the printhead, so that no more toner is developed while the copy sheet continues to run through normally. The result is an unfinished print. This is undesirable.
Such undesirable situations will occur less rapidly by raising the threshold value at which the printing operation is temporarily interrupted, so that the supply of toner, in between the production of prints, takes place earlier. However, if the threshold is raised, prints will be produced in direct succession to a lesser degree so that productivity falls off. This is also undesirable.